In recent years, various types of displays are employed for all sort of applications such as portable devices, personal computers, monitors or televisions. Among them, liquid crystal displays are widely used for various applications which include miniaturized products for portable devices and which recently have spread to larger products such as monitors and televisions. The liquid crystal display itself is not a luminous object, and it becomes possible to use it as a display by introducing light from the backside with a backlight.
It is required for the backlight not only to simply irradiate light, but also to illuminate the entire image uniformly and brightly. Thus, an optical functional sheet such as a light diffusive sheet or a prism sheet is commonly added for the purpose of uniformly brightening the backlight. More specifically, the backlight commonly comprises a light diffusive sheet that equalizes the distribution of light emission across the light guiding plate, and additionally the prism sheet is superposed to collect light in a normal direction to the sheet surface for the purpose of improving brightness.
The prism sheet is a sheet having a configuration in which a number of prisms having a generally triangular cross section are arranged, and the use of this sheet effectively collects light from the backlight and directs it in the normal direction to the sheet surface, thereby improving brightness in the normal direction to the sheet surface (see, for example, U.S. Pat. No. 5,161,041).
However, since the columns of prisms located on the surface of the prism sheet are very delicate and have a conformation of sharp apexes, the prism sheet may readily scratch the surface during production or handling, and thus provides a drawback of deteriorating the quality of images. In addition, an additional light diffusive sheet having lower diffusivity is used for the purpose of hiding the lines of prism columns, thereby increasing the number of sheets used for the backlight.
In addition, further improvements in performance and effectiveness or further improvements in reducing thickness and weight are required for the optical functional sheet used for these backlights, and it is useful for achieving these requirements to integrate functions such as applying a functional layer on the surface and sticking another sheet thereon, and the like. However, the surface treatment is not available for the type of sheet such as, typically, a prism sheet, which performs its function as a result of its surface configuration.
The present inventors have found, as a result of their various investigations carried out for the purpose of solving the problems indicated above, a sheet having a function of internally collecting light which is not derived from its surface configuration, and thus the present invention has been achieved.
An object of the present invention is to provide an optical functional sheet which is capable of providing a function of collecting light due to its internal configuration without utilizing any function of collecting light due to its surface configuration, and which can significantly improve the brightness in a normal direction to the sheet surface in the case of being used for applications such as a backlight for liquid crystals. Further, it is an object to present a conjugated optical functional sheet having the function of collecting light as well as other functions.